Numeric Arguments
All emacs commands can take a numerical argument. For instance, to
kill 5 lines you can do M-5 C-k or C-u 5 C-k. If no number
follows C-u then a default numerical argument of 4 is used.
Where it makes sense, negative numerical arguments are accepted. Some commands
don't interpret the numerical argument as a request for repetion but as
a flag to modify behaviour. E.g. C-<Space> sets a mark whereas
C-u C-<Space> goes back through previous marks.